1. Field of the Invention
The present invention relates to a glass substrate for a magnetic disk and a magnetic disk apparatus.
2. Description of the Related Art
Nowadays, drastic technical innovations are needed in information recording technologies, especially in magnetic recording technologies as information technologies (IT) evolve. To catch up the trend, fast-paced increase in information recording density continues for a magnetic disk that is mounted on a hard disk drive (HDD) functioning as a magnetic disk apparatus that serves as a computer storage.
Recently, there is an increasing demand for mounting an HDD in a portable device. To satisfy such a demand, a glass substrate with high strength, high rigidity, and high impact-resistance is employed as a substrate for a magnetic disk. A glass substrate can easily provide a smooth surface, so that a fly height of a magnetic head that performs recording and reproducing while flying over a magnetic disk can be reduced. Therefore, a magnetic desk with higher information recording density is attainable by the use of the glass substrate as a magnetic disk substrate. Namely, the glass substrate is advantageous in that it reduces the fly height of the magnetic head.
To increase the information recording capacity in the magnetic disk, an information signal-unrecorded area of the magnetic disk needs to be reduced. In achieving this, introduction of a load/unload method (an LUL method) otherwise known as “a ramp load method” that enables increasing the information recording capacity has been promoted in place of a conventional contact start stop method (a CSS method) as a start/stop method for an HDD.
The conventional CSS method disadvantageously needs to have a CSS zone in a magnetic disk, on which a magnetic head is placed when the magnetic disk is not in use (in a stop state).
By contrast, in the LUL method, the magnetic head moves toward an outer periphery of the magnetic disk and then it stops at a position that is out of a space over the magnetic disk when the magnetic disk is not in use, thus enabling to prevent contact of the magnetic head with the magnetic disk unlike the CSS method. This can eliminate the need to provide antistick concavities and convexities on the surface of the magnetic disk that have been generally provided in the CSS zone. By the LUL method, a highly smooth main surface of the magnetic disk is attainable.
Thus, further reduction of the fly height of the magnetic head can be realized by the LUL magnetic disk compared with the CSS magnetic disk, so that a signal noise ratio (an S/N ratio) of recording signals can be improved and higher recording density is attainable.
The introduction of the LUL method has enabled a narrower fly height for the magnetic head; however, this created another requirement. That is, a stable operation of the magnetic head is required at a nanosized fly height of equal to or less than 10 nanometers. In such a minute space, fly stiction phenomenon frequently occurs when the magnetic head flies over the magnetic disk.
The fly stiction phenomenon is an unstable fly height or flying state of a magnetic head that is flying over a magnetic disk, thus, generating irregular reproduction output fluctuation. This fly stiction phenomenon may cause head crash, that is, bumping the magnetic disk by the flying magnetic head occurs.
Efforts have been made for the conventional HDDs to prevent such fly stiction phenomenon by applying a higher rotation velocity to the magnetic disk, thus applying a higher relative linear velocity between the magnetic disk and the magnetic head, and by stabilizing the fly height or flying state by the structure of the magnetic head.
However, recently, demand has been increasing for a smaller HDD mountable on devices such as a cell phone, a digital camera, a portable information device, and a car navigation system that have a much smaller device size than a personal computer (PC) and need a high response speed. For example, the small-sized HDD that accommodates a magnetic disk manufactured using a substrate with an outer diameter of equal to or less than 50 millimeters and a thickness of equal to or less than 0.5 millimeter is needed.
A small magnetic disk of which outer diameter is equal to or less than 50 millimeters is typically used for a small HDD. In the small magnetic disk, an outer circumference and an inner circumference are small, so that the relative linear velocity between the magnetic disk and the magnetic head is low. In addition, a small spindle motor is generally used to rotate the small magnetic disk. Because the spindle motor is small, further speeding-up of the rotation of the magnetic disk is not easy, which may influence the stability of the fly height and flying state of the magnetic head, or may not sufficiently prevent occurrence of the fly stiction.
A small magnetic head is used in the small magnetic disk. The stability of the fly height or flying state of the magnetic head is low.
In addressing prevention of the fly stiction phenomenon, for example, Japanese Patent Application Laid-open No. 2005-317181 discloses to increase the surface roughness in a radial direction of a main surface of a circular substrate by forming an anisotropic texture on the main surface in the generally circumferential direction. On the other hand, Japanese Patent Application Laid-open No. 2007-12157 teaches to use a disk substrate having a diameter of equal to or less than 1 inch and having a relation represented by 0 nanometer<Ra1−Ra2≦0.2 nanometer, where Ra1 is an average surface roughness of an inner circumferential surface of data area and Ra2 is an average surface roughness of an outer circumferential surface of the data area.
As stated above, the technologies in Japanese Patent Application Laid-open No. 2005-317181 and Japanese Patent Application Laid-open No. 2007-12157 have addressed the stable floating characteristics of the magnetic head by increasing the average surface roughness of a glass substrate for a magnetic disk in a circumferential direction from an outer circumferential side to an inner circumferential side of a main surface of the glass substrate. The arithmetic average roughness means the arithmetically averaged roughness of the surface of the glass substrate for the magnetic disk measured by causing a measuring probe to scan the glass substrate in the circumferential direction when a 5-micrometer×5-micrometer area of the glass substrate is measured by an atomic force microscope.
However, there has been a problem that fly stiction phenomenon in a further downsized magnetic disk could not be surely prevented only by the conventional controlling of the average surface roughness in the circumferential direction of the substrate.
In particular, in a recent magnetic disk with a fly height of less than 10 nanometers, air molecule is in the order of a mean free path (64 nanometers) of a typical air molecule. Thus, the generation of a floating pressure can not be explained by the flow of an airflow continuum. The influence of collision of air molecules with a solid wall is larger than that of a viscosity resistance generated due to collision between air molecules and air molecules.